Diffusional release of drug particles from a polymer matrix is an important and commonly used method of achieving controlled drug release. The release of drug particles from a polymer matrix is thought to occur through a network of interconnected pores, which are created by the drug particles that are initially loaded in the matrix. In particular, an aqueous medium imbibes into the matrix and dissolves the drug particles. The drug particles, once dissolved, leave behind pores in the polymer matrix and the drug particles elute through these pores. Such a release mechanism, however, is affected by particle size and loading. Specifically, drug particles may not touch each other when the drug loading is low or when the drug particles are small. Thus, with low loading or small particle size, many drug particles may be completely surrounded by polymer resulting in the drug particles being trapped in the polymer. Therefore, only those drug particles on or having a path to the surface of the matrix will be able to be released.
One method of forming a porous polymer without relying on the drug particles to create the pores is by cross-linking linear chains of monomers, such as styrene and divinylbenzene, to create very small pores within the polymer. Cross-linking, however, may have to be reduced to increase pore size. Such reduction in cross-linking may decrease the physical stability of the polymer such that the polymers cannot withstand much pressure before collapsing.
Another method of forming a porous polymer is using leachable additives to create the pores in the polymers. Leaching, however, is considered an undesirable attribute since leaching cannot be controlled and may continue after the device containing the porous polymer is deployed into the body.
Another method of forming a porous polymer where the pores are formed independent of cross-linking is to polymerize monomers in the presence of porogens, which are soluble in monomers but insoluble in formed polymers. As polymerization proceeds, pores are formed in the spaces where porogens are found. Porogens, however, may plasticize the surrounding polymer and can leach into the body thereby raising biocompatibility and toxicity concerns.
Yet another method of forming a porous polymer involves polymerizing monomers as the continuous phase in a high internal phase emulsion or foam to form cross-linked homogenous porous polymers. Emulsions and foams, however, are hard to control and the emulsion and foaming agents need to be in small quantities in order to maintain integrity.
Therefore, there is a need in the art for a method of manufacturing a porous coating for a medical device that provides greater flexibility in the amount and size of drug particles that can be loaded in the polymer matrix, provides for an improved kinetic drug release, and/or provides greater compressional strength for the porous coating.